Nonlinear semiconductor switches such as diodes and the like are employed in various electronic control applications where it is necessary to switch current flow in response to a prescribed level of voltage. One such application involves visual display devices which employ a matrix of electrically energizable pixels (picture elements). The pixels may consist, for example, of well known liquid crystal elements in which a film or cell of liquid crystal material is positioned between a pair of electrodes. The liquid crystal material has the structural characteristics of cybotatic liquids but is considerably more viscous and exhibits much more evidence of structure. Smectic and nematic type liquid crystals, which are commonly used in displays, consist of elongate molecules whose longitudinal axes are rotated when an electric field of prescribed magnitude is applied to the material. The molecular rotation either blocks or allows transmission of light through the liquid crystal material, thereby altering its optical properties.
One common type of liquid crystal display employs twisted nematic crystals sandwiched between a pair of parallel glass sheets and a pair of polarizers respectively on opposite sides of the glass sheets. The glass sheets have parallel lines formed by etching or the like on the opposing faces thereof. The molecules of the nematic crystal material near the surface of each glass plate tend to align their long axes parallel to the lines on the glass plates. The plates are oriented relative to each other such that the two sets of lines are non-parallel, e.g. 90 degrees off axis, thus giving the nematic crystals a helical or twisted orientation which prevents the transmission of light through the cell. Application of an electric field across the cell rotates or "untwists" the molecules so that their axes extend substantially parallel to each other, thus allowing passage of light through the cell.
In order to display changeable data using liquid crystal displays, the pixels are arranged into M rows and N columns defining a matrix array that is addressed using conventional "X-Y" addressing techniques which employ M+N address lines. Each pixel possesses a unique X-Y location in the matrix which may be addressed by a corresponding combination of X and Y addressing lines.
The magnitude of the "threshold" voltage at which a liquid crystal pixel is switched to a different optical state is relatively low. In the case of matrix arrays where the pixels are closely spaced, a significant level of electrical cross talk exists in the addressing circuitry between adjacent pixels. Cross-talk having a signal strength as low as one-third of the pixel threshold voltage may be sufficient in some cases to energize pixels which are not intended to be addressed, consequently it is necessary to provide means for isolating each pixel to some degree from circuit cross-talk, thereby improving the electrical isolation between adjacent pixels. To achieve isolation in the past, nonlinear devices such as diodes have been placed in series with the liquid crystal pixels to increase the effective threshold of the pixels and thereby block stray currents emanating from addressed pixels from energizing adjacent, non-addressed pixels.
Addressing circuitry is complicated by the fact that an applied electric field of one sustained polarity (D.C.) results in electrolytic degradation of the liquid crystal material. Accordingly, it is necessary to periodically reverse the polarity of the applied field using conventional switching techniques. Polarity reversal is usually effected for each frame of the display, i.e. each time the matrix is scanned. One typical circuit for providing AC excitation of a liquid crystal display matrix is shown in U.S. Pat. No. 3,654,606 issued Apr. 4, 1972 to Marlowe, et al; a pair of X or Y address lines per pixel are employed, one of positive polarity and the other of negative polarity, and an AC signal is applied to a third address line. Each pixel is connected to the pair of address lines through associated reversed biased, blocking diodes.
The use of conventional diodes as isolating devices for pixel addressing has been less than completely satisfactory for several reasons. First, it is not always possible to control or select the threshold voltage at which the diode commences to conduct current when forward biased, and in any event, variations may exist in the threshold voltge of the diodes used in a single matrix. For a given level of addrress signal voltage, some of the forward biased diodes may conduct while others having a higher threshold voltage may not conduct or may conduct at a different rate. Accordingly, it is necessary to assure that a relatively high level address signal is applied to address the pixels.
It is also desirable to minimize the rise time of the current applied to each pixel in order to quickly switch the pixel after it has been addressed. The current-voltage characteristics of diodes previously employed for pixel isolation are such that the rise time of the current conducted by the diode following the application of threshold voltage is slower than desired.